Shield for microstrip circuits



July 10, 1956 J. P. ADAMS 2,754,484

SHIELD FOR MICROSTRIP CIRCUITS Filed Nov. 22, 1954 V; emu/v0 0/ELECTR/C4) coNowmR) RECEIVER TRANSMITTER M/c/aos TRIP MIC/P08 TRIP COMPONENTS compo/mum IL! I 11,, I 1,111,, 111111111111111111111 IIIIIIIIIIIIIIII I717 n I 4 \\KI} 7 M TRANS. 3 4 i L I I l I 1 j 4 2 REC. L/

INVENTOR JOHN R ADAMS ATTORNEY United States Patent SHIELD FOR MICROSTRIP CIRCUITS John Pershing Adams, Amityville, N. Y., assignor to International Telephone and Telegraph Corporation, New York, N. Y., a corporation of Maryland Application November 22, 1954, Serial No. 47%),383

4 Claims. (Cl. 333-9@) This invention relates to an electrical communication system of the parallel conductor, line-above-ground type, and particularly to shielding means utilized in such systems to prevent spurious coupling between independent circuits constituting the system.

In co-pending application of D. D. Grieg-H. F. Engelman, Serial No. 227,896, filed May 23, 1951, and Serial No. 234,503, filed June 30, 1951, now Patent No. 2,721,312, granted October 18, 1955, the line-aboveground transmission system, hereinafter called microstrip transmission system, was disclosed. The microstrip line comprises a line conductor and a planar ground conductor spaced a given wavelength fro-m the line conductor; the planar dimensions of the ground conductor are considerably greater than the corresponding dimensions of the line conductor. The conductors are spaced apart by a dielectric through which the energy is transmitted. By making the ground conductor considerably wider than the line conductor, an image effect is produced which provides in effect, an electric and magnetic field distribution between the two conductors. As described in detail in the above-mentioned application, the diameteror width of the line and the spacing between the line and the planar conductor, determine the characteristic impedance of the transmission path. The spacing is preferably a fraction of one-quarter wavelength of the operating frequency which is usually less than the width of the line conductor.

In the use of microstrip line transmission circuits, dimculties sometimes arise when it is desired to place in close physical relationship, a sensitive receiver circuit and a relatively high powered circuit, such as a transmitting circuit. One of the advantages of the microstrip line transmission system is that it permits the use of a single ground plane which is generally in the form of a plate, with a large number of circuit elements. Thus, on the single metal plate constituting the ground plane, there may be provision for a receiver and intermediate frequency (I. F.) amplifier, oscillator, transmitter, etc.

Where different power levels are encountered in adjacent circuits, there is a certain amount of energy transferred from one circuit to the other by spurious coupling. Although the circuits can usually be separated by a sufficient distance to maintain the spurious coupling at tolerable levels, in some instances, this is not possible because of a desire or necessity to conserve space. In these cases, physical shielding in the form of metal covers can be used, with the covers electrically connected to the ground plate. Although this method of shielding is generally satisfactory, effort has been made to simplify the shielding means and to eliminate the shielding covers wherever possible.

Accordingly, it is an object of this invention to simplify the known techniques of shielding by eliminating the number of covers required to shield one adjacent circuit from another while at the same time maintaining undesired coupling at a minimum.

In accordance with an aspect of the invention, two independent circuit assemblies, such as a transmitter and a g, 2,754,484 Patented July 10, 1956 receiver, are shielded from one another by arranging the circuits physically so that the portions which would cause electrical interference by coupling are disposed on opposite sides of the metal plate forming the ground plane. The ground plate section of each circuit is arranged adjacent to the circuit elements of the microstrip line of the adjacent circuit. A continuous circuit is formed by interconnecting the microstrip line elements, and interconnecting the ground plates.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a plan view of a microstrip receiver and transmitter, the components of which are shown in block diagram;

Fig. 2 is a view taken along lines AA of Fig. 1, assuming that the microstrip circuits are assembled in the conventional manner; Fig. 3 is a view taken along lines AA of Pig. 1, with the microstrip line being assembled in accordance with the invention and mounted in a closed container; and

Fig. 4 is a front elevational view of Fig. l, with the microstrip line being assembled in accordance with the invention.

Referring now to Fig. 1, there is shown in plan view a typical microstrip circuit comprising two adjacent independent circuit assemblies, for example, a receiver and a transmitter. The microstrip transmission line comprises a ground plate conductor 1, a line conductor, including microstrip components, shown by block diagram at 2 and 3, for the receiver and transmitter respectively, and a dielectric 4- intermediate the ground and line conductors. A microstrip radio receiver, including the various components is described in detail in co-pending application Serial No. 317,206, filed October 28, 1952. However, since the structural details of the various microstrip components form no part of the present invention, the components, including the line conductors, have been shown in block diagram.

Referring to Fig. 2, there is shown in cross-section, an end view of the microstrip line, taken along lines AA of Fig. 1, assuming that the microstrip circuits shown in Fig. l are mounted in accordance with the conventional technique; i. e., arranging the circuit components on the same side of the ground plate 1. The ground plane, and the dielectric layer 4, generally are common to both circuits.

It is to be realized that some of the circuit elements extend below the ground plane but customarily, these are shielded elements such as coaxial connectors, crystal holders, oscillator cavities, etc. The outsides of the metallic shielding covers are connected to the ground plate and hence, are at the same potential.

Figs. 3 and 4 show the same circuit assemblies mounted in accordance with the present invention.

Referring first to Fig. 3, the microstrip line is shown with the transmitter circuit arranged similarly as the transmitter circuit shown in Fig. 2. However, the receiver is mounted on the opposite side of the ground plate. Thus, the circuit elements 2 of the receiver and ground plate 1 of the transmitter are mounted on one side of the dielectric strip 4 and the ground plate 1 of the receiver and the circuit elements 3 of the transmitter are mounted on the opposite side of the dielectric 4. The ground plates are electrically connected by any suitable means such as a wall 5, or at frequent intervals by straps which penetrate the dielectric layer 4 and connect the two otherwise separated ground plates 1. The utility of the in- Vention is particularly apparent when it is desired to mount the microstrip line in a closed container 7 which may be of rectangular or curvilinear cross section. For example, the container 7 may be a cylindrical disc or a rectangular box madeof conductive material. In mounting the microstrip line in the container, the ground conductor is positioned to divide the container into two parts. The receiver components are mounted on one side of the conductor and the transmitter components are mounted on the opposite side. Thus, in this application, it is readily seen that the circuit requires no additional shielding for effectively shielding the receiver components from the transmitter components.

Fig. 4 shows the interconnection of the receiver and transmitter by any suitable means such as a link 6 which also penetrates the dielectric layer 4 to make the connection. This connection is made by cutting away the ground plate -1 from the connecting link 6. A plurality of common receiver-transmitter connections may be made in this manner.

It is to be realized that although a common dielectric layer and two ground plates are shown in the drawing, two separate dielectric layers and a common ground plate may be used without departing from the scope and spirit of the invention. Moreover, the connecting means between the ground plates, and between the two microstrip line circuits may take any form, providing it performs the desired function as described above.

While I have described above the principles of my invention in connection withspecific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation of the scope or" my invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

1. In a microwave electrical communication system of the parallel conductor, line-above-ground type, in which the ground is a planar conductor and the electrical energy is transmitted in the space between the conductors, at least two independent circuit assemblies mounted in close physical proximity with each other, a layer of dielectric material intermediate each of said parallel conductors, means mounting the line conductor of one circuit and the planar conductor of the other circuit on the same side of the dielectric'layers, and means mounting the planar conductor of one circuit and the line conductor of said other circuit on the other side of said dielectric layers, means coupling said planar conductors together, and electric shielding means completely enclosing said circuit assemblies and connected to said planar conductors at opposite ends thereof to prevent spurious coupling between said circuit assemblies.

2. The system according to claim 1 wherein said dielectric layers comprise a common dielectric layer intermediate the conductors of both said circuits.

3. The system according to claim 1 wherein said independent circuit assemblies comprise a receiver circuit and a transmitter circuit, and further comprising means for coupling said circuits together.

4. In a microwave electricalcommunication system of the parallel conductor, line-above-ground type, in which the ground is a planar conductor and the electrical energy is transmitted in the space between the conductors, a receiver circuit, a transmitter circuit, a layer of dielectric material intermediate said parallel conductors and common to both said circuits, means mounting the line conductor of the receiver and the ground conductor of the transmitter on one side of the dielectric, and means mounting the ground conductor of the receiver and the line conductor of the transmitter on the other side of said dielectric, means penetrating said dielectricfor electrically connecting said ground conductors together, and means penetrating the dielectric for connecting the receiver to the transmitter. 

